The present invention relates to an average-frequency measuring device for indicating an accurate frequency by measuring the frequency of an input signal for a plurality of times and then averaging the measured frequencies.
For measuring a carrier frequency of a so-called burst signal such for example as radar pulses in which a carrier signal is 100% amplitude-modulated by pulses, cycles of the carrier wave are counted over a plurality of burst signals to increase the measurement accuracy. If a measurement period if fixed, however, the accuracy of measurement of frequencies varies dependent on the length of the burst signal and the repetition frequency.
To cope with the above problem, there has been proposed a frequency measuring device for indicating measured frequencies having a desired predetermined measurement accuracy in measuring a frequency of a certain portion of a repetitively swept frequency signal, and other carrier frequencies of repetitive signals, as disclosed in U.S. Pat. No. 4,468,614. According to the proposed device, when a measurement process is started, the length .tau. of a count period in one burst signal in an input signal is calculated, a required net counting period is calculated from the length .tau. and a measurement accuracy setting R, and the input signal and a clock signal is counted until the sum of count periods in respective burst signals reaches the net counting period. Any measured frequency value is not displayed unless the counting of the net counting period is completed. In case the width of the burst signals is small and the repetition frequency is low as with radar pulses, it takes a considerable length of time before a measured result becomes available after the measurement has started. Since the above cycle is successively repeated for measurement, the measured results are renewed only upon elapse of each net counting period, even when the input signal is continuously measured. In the proposed device above, the block signal is counted by a counter circuit, and the count is used to detect when the net counting period reaches its calculated value, that is, when the counting is finished. Where such a process is monitored at all times by a microcomputer, the microcomputer is subjected to a large burden. The detection of such a count ending time is therefore implemented by hardware. However, the hardware arrangement is complicated since the counter circuit has many bit positions.
In view of the foregoing, it may be possible a divide-by-10.sup.n frequency divider could be employed, i.e. a 1/10.sup.n -frequency divider, as the counter circuit, to determine the frequency-division ratio n so that the period, from when the frequency divider starts the frequency division until an output is produced from the frequency divider, is longer than the calculated net counting period, and the microcomputer is then interrupted when the frequency divider output is issued, so as to measure the frequency of an input signal from the count value of the input signal at the time. Such an arrangement would be quite simple in construction since the microcomputer would not be required to monitor the clock signal count in the counter at all times and also since there would be no need for hardware for detecting a count ending time because the end of the counting is detected when the frequency divider output is issued. However, the net counting period would be such as 0.01 second, 0.1 second, 1 second, 10 seconds, or 100 seconds, for example. If the calculated net counting period were 12 seconds, for example, the period required for frequency division up to when a frequency divider output is produced after the frequency divider has started its operation would be required to be 100 seconds. After the counting period of 12 seconds necessary for producing a measured value having a respective measurement accuracy setting has elapsed, another counting period of 88 seconds would be needed. Since the counting period would be a net period, and a long period free from any counting operation would be present between adjacent burst signals, a long and wasteful period of time would be consumed before a measured value would be displayed.